1. Field of the Invention
The present invention relates to a compound semiconductor material having a large band gap which can be effectively used for a short wavelength light-emitting element, a semiconductor element using the same, and a method of manufacturing the semiconductor element.
2. Description of the Related Art
With an increase in speed and recording density of a data processing system, demands have arisen for short wavelength semiconductor lasers. As conventional materials for semiconductor lasers, Group III-V semiconductor materials such as AlGaAs, InGaAsP, and AlInP have been used. However, all these materials have band gaps whose values are far from satisfying the requirements of green emission. Green emission cannot be realized by using existing materials, and hence material design based on a new concept is required.
When Group III-V compound semiconductor materials are considered in view of large band gaps, nitrides and phosphides of light Group III elements have large band gaps, e.g., BN (4 or 8 eV), AlN (6 eV), GaN (3.4 eV), InP (2.4 eV), AlP (2.4 eV), and GaP (2.3 and 2.8 eV). Of these materials, BN has a large band gap, but a high-pressure phase (c-BN) of BN having a 4-coordinate (sp3) bond is difficult to synthesize. In addition, there are three types of BN. and mixtures of BN and other elements are easily formed. For these reasons, BN cannot be used in practice. Moreover, impurity doping in BN is difficult. InN has a relatively small band gap and poor thermal stability. Generally, only polycrystals can be obtained from InN. The band gaps of both AlP and GaP are slightly insufficient. Other compounds, i.e., AlN and GaN have large band gaps and are excellent in stability. Therefore, AlN and GaN are suitable for short wavelength light-emitting elements. However, AlN and GaN have Worzeite type (to be referred as a WZ type) crystal structures. In addition, since GaN has a strong ionization tendency, lattice defects tend to occur. Therefore, a p-type semiconductor having a low resistance cannot be obtained from GaN.
In order to solve these problems, attempts have been made to obtain materials whose band gaps are increased by mixing B and N with Group III-V compounds containing no B and N which have been developed as conventional materials for semiconductor lasers. However, there are large differences in lattice constant between the conventional materials and materials containing B and N, ranging from 20 to 40%. In addition, they have different lattice configurations. For these reasons, stable crystals have not been obtained yet by mixing B and N with III-V compounds containing no B and N. For example, if N is mixed with GaP, the content of N cannot exceed 1% with respect to GaP. Hence, a sufficiently large band gap cannot be obtained.
According to the studies of the present inventors, the essential reasons why a p-type crystal having a low resistance cannot be obtained by using GaN or AlN are that defects tend to occur due to the strong ionization tendency and that these compounds have WZ structures instead of Zinc Blend type (to be referred to as a ZB type hereinafter) structures. Such a situation will be described below with reference to FIGS. 1A and 1B.
FIG. 1A shows a band structure of a cubic semiconductor. In this case, the .GAMMA. point is set at the bottom of a conduction band for convenience. However, even if it is set at another position, there will be no substantial difference in discussion. Degeneracy of bands of heavy and light holes occurs near the apex of a valence band of interest. In addition, an orbit which is shifted to the low energy side due to spin-orbit interaction is present in the valence band. In this case, since holes are present both in the heavy and light bands, the average mass of the holes in the two bands is regarded as an effective mass. The WZ structure of a hexagonal crystal, however, is greatly influenced by a crystalline field due to its strong uniaxial isotropy. As shown in FIG. 1B, therefore, degeneracy of the bands of heavy and light holes is released and the band of heavy holes is shifted to the high energy side. As a result, the holes are present in the band of heavy holes. Since an acceptor energy level becomes deep with an increase in effective mass of the holes, holes tend not to be emitted. For this reason, low-resistance p-type semiconductors cannot be formed.
As described above, no conventional semiconductor material can satisfy the conditions required for realizing a green emission semiconductor laser or a high-luminance blue LED, i.e., having a sufficiently large band gap, e.g., 2.7 eV, allowing easy p-n junction control, and being excellent in crystal quality. Although AlN and GaN are nitrides which are effective in obtaining large band gaps, low-resistance p-type layers cannot be obtained.